eth2.0-specs/specs/merge/beacon-chain.md

# Ethereum 2.0 The Merge

**Warning:** This document is based on [Phase 0](../phase0/beacon-chain.md) and considered to be rebased to [Altair](../altair/beacon-chain.md) once the latter is shipped.

**Notice**: This document is a work-in-progress for researchers and implementers.

## Table of contents

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- [Introduction](#introduction)
- [Constants](#constants)
  - [Transition](#transition)
  - [Execution](#execution)
- [Containers](#containers)
  - [Extended containers](#extended-containers)
    - [`BeaconBlockBody`](#beaconblockbody)
    - [`BeaconState`](#beaconstate)
  - [New containers](#new-containers)
    - [`Transaction`](#transaction)
    - [`ApplicationPayload`](#applicationpayload)
- [Helper functions](#helper-functions)
  - [Misc](#misc)
    - [`is_transition_completed`](#is_transition_completed)
    - [`is_transition_block`](#is_transition_block)
  - [Block processing](#block-processing)
    - [Application payload processing](#application-payload-processing)
      - [`get_application_state`](#get_application_state)
      - [`application_state_transition`](#application_state_transition)
      - [`process_application_payload`](#process_application_payload)

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## Introduction

This is a patch implementing executable beacon chain proposal. 
It enshrines application execution and validity as a first class citizen at the core of the beacon chain.

## Constants

### Transition
| Name | Value |
| - | - |
| `TRANSITION_TOTAL_DIFFICULTY` | _TBD_ |

### Execution

| Name | Value |
| - | - |
| `MAX_BYTES_PER_TRANSACTION_PAYLOAD` | `2**20` |
| `MAX_APPLICATION_TRANSACTIONS` | `2**14` |
| `BYTES_PER_LOGS_BLOOM` | `2**8` |


## Containers

### Extended containers

*Note*: Extended SSZ containers inherit all fields from the parent in the original
order and append any additional fields to the end.

#### `BeaconBlockBody`

*Note*: `BeaconBlockBody` fields remain unchanged other than the addition of `application_payload`.

```python
class BeaconBlockBody(phase0.BeaconBlockBody):
    application_payload: ApplicationPayload  # [Added in Merge] application payload
```

#### `BeaconState`

*Note*: `BeaconState` fields remain unchanged other than addition of `application_state_root` and `application_block_hash`. 


```python
class BeaconState(Container):
    # Versioning
    genesis_time: uint64
    genesis_validators_root: Root
    slot: Slot
    fork: Fork
    # History
    latest_block_header: BeaconBlockHeader
    block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
    state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
    historical_roots: List[Root, HISTORICAL_ROOTS_LIMIT]
    # Eth1
    eth1_data: Eth1Data
    eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH]
    eth1_deposit_index: uint64
    # [Added in Merge] hash of the root of application state
    application_state_root: Bytes32
    # [Added in Merge] hash of recent application block
    application_block_hash: Bytes32
    # Registry
    validators: List[Validator, VALIDATOR_REGISTRY_LIMIT]
    balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT]
    # Randomness
    randao_mixes: Vector[Bytes32, EPOCHS_PER_HISTORICAL_VECTOR]
    # Slashings
    slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR]  # Per-epoch sums of slashed effective balances
    # Attestations
    previous_epoch_attestations: List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH]
    current_epoch_attestations: List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH]
    # Finality
    justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH]  # Bit set for every recent justified epoch
    previous_justified_checkpoint: Checkpoint  # Previous epoch snapshot
    current_justified_checkpoint: Checkpoint
    finalized_checkpoint: Checkpoint
```

### New containers

#### `Transaction`

Application transaction fields structured as an SSZ object for inclusion in an `ApplicationPayload` contained within a `BeaconBlock`.

```python
class Transaction(Container):
    nonce: uint64
    gas_price: uint256
    gas_limit: uint64
    recipient: Bytes20
    value: uint256
    input: List[Bytes1, MAX_BYTES_PER_TRANSACTION_PAYLOAD]
    v: uint256
    r: uint256
    s: uint256
```

#### `ApplicationPayload`

The application payload included in a `BeaconBlock`.

```python
class ApplicationPayload(Container):
    block_hash: Bytes32  # Hash of application block
    coinbase: Bytes20
    state_root: Bytes32
    gas_limit: uint64
    gas_used: uint64
    receipt_root: Bytes32
    logs_bloom: Vector[Bytes1, BYTES_PER_LOGS_BLOOM]
    difficulty: uint64  # Temporary field, will be removed later on
    transactions: List[Transaction, MAX_APPLICATION_TRANSACTIONS]
```

## Helper functions

### Misc

#### `is_transition_completed`

```python
def is_transition_completed(state: BeaconState) -> Boolean:
    state.application_block_hash != Bytes32()
```

#### `is_transition_block`

```python
def is_transition_block(state: BeaconState, block_body: BeaconBlockBody) -> boolean:
    return state.application_block_hash == Bytes32() and block_body.application_payload.block_hash != Bytes32()
```

### Block processing

```python
def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    process_operations(state, block.body)
    process_application_payload(state, block.body)  # [New in Merge]
```

#### Application payload processing

##### `get_application_state`

*Note*: `ApplicationState` class is an abstract class representing ethereum application state.

Let `get_application_state(application_state_root: Bytes32) -> ApplicationState`  be the function that given the root hash returns a copy of ethereum application state. 
The body of the function is implementation dependent.

##### `application_state_transition`

Let `application_state_transition(application_state: ApplicationState, application_payload: ApplicationPayload) -> None` be the transition function of ethereum application state. 
The body of the function is implementation dependent.

*Note*: `application_state_transition` must throw `AssertionError` if either the transition itself or one of the post-transition verifications has failed.

##### `process_application_payload`

```python
def process_application_payload(state: BeaconState, body: BeaconBlockBody) -> None:
    """
    Note: This function is designed to be able to be run in parallel with 
    the other `process_block` sub-functions
    """

    if is_transition_completed(state):
        application_state = get_application_state(state.application_state_root)
        application_state_transition(application_state, body.application_payload)

        state.application_state_root = body.application_payload.state_root
        state.application_block_hash = body.application_payload.block_hash

    elif is_transition_block(state, body):
        assert body.application_payload == ApplicationPayload(block_hash = body.application_payload.block_hash)
        state.application_block_hash = body.application_payload.block_hash
    
    else:
        assert body.application_payload == ApplicationPayload()
```